This invention relates to a process for preparing a polymer emulsion containing colloidally suspended therein an interpenetrating polymer network wherein a first polymer network is intertwined on a molecular scale with a second polymer network and optionally additional polymer networks. The polymer emulsion of this invention is useful as binder of fibers of fabrics, especially fiberfill.
Fiberfill is a generic term used to describe a variety of nonwoven fabrics for a variety of end uses. The common feature of all fiberfill products is a measure of loft or thickness in the fabric. This loft is a characteristic of value because it imparts insulation to outerwear and bed quilt stuffing, cushioning in furniture padding, dust holding capacity to filter media and resiliency to scrubbing pads. The most common construction of a fiberfill product is a loosely garnetted, cross-lapped or air laid web of 6 to 30 denier polyester staple fibers which is bonded (locked in its particular fiber arrangement) by an emulsion polymer binder. Fiberfill products can be made with other fibers, e.g. polyamide, cellulose acetate, rayon, glass, alone or in blends with each other. Some fiberfill is sold without a bonding agent but the material will lack durability, tensile strength and resiliency when compared to a bonded product. Bonding methods other than emulsion polymers, such as needle punching, and meltable fibers and powders are also used, but the polymer emulsion method produces the optimum strength/loft ratios for the majority of fiberfill markets.
The polymer emulsion product used as the binder is usually one of the following chemical types: polyvinylacetate, acrylic copolymers, styrene-butadiene copolymers or polyvinylchloride. Polyvinylacetate is the most common binder and in recent years it has been made white enough and strong enough to replace most of the acrylic polymer traditionally used. Polyvinylchloride is used where flame resistance is of prime concern and styrene-butadiene copolymers are used for special rubbery applications.
The characteristic of initial loft is unaffected by the chemical type of the binder used. However, initial loft is not the loft of value. Fiberfill products in their normal use are compressed, reducing the initial loft, and released many times. The true value of loft is how thick the fiberfill web is after repeated compression/recovery cycles. One drawback of current polymer bonded fiberfill technology is that temperatures over 100.degree. F. will soften the binder and cause the fiberfill product to permanently lose loft if it is compressed at this elevated temperature. Temperatures of up to 180.degree. F. are encountered in the shipping and use of many fiberfill products. Typically a fiberfill product, which may lose only 15% of its initial loft if compressed and released at 80.degree. F., will lose more than 80% of its loft if tested the same way at only 120.degree. F. Higher temperatures are expected to even more dramatically damage this loft recovery.
Another area in which polymer binders are useful is in the manufacture of glass fiber mats. Nonwoven fiber mats made from glass staple fibers are finding uses in many markets.
Glass fibers themselves are known for their high tensile strength and inertness to reaction with other materials. Since they are generally not available in crimped form, these features make bonding the glass fiber mats into integral networks of adequate strength, flexibility and toughness a difficult task. This bonding is customarily accomplished with polymer emulsions or other thermosetting resins such as urea formaldehyde, melamine formaldehyde and phenol formaldehyde resins and the like.
Insulation products require a maximum dead air space per unit weight. This must not only be achievable in the initial production, but be retained after the insulation product has been compressed for storage and shipment and subsequently placed in service as insulation in a building or machinery construction. The binding chemical in this case must not distort or deform in various handling operations. To date, urea-formaldehyde resins have provided this binding function most economically.
Nonwoven fabrics cover a wide array of products including consumer goods like mattress dust shields, disposable diaper cover fabrics, cleaning towels, carpets, draperies and industrial and commercial goods like wipe cloths, tire cords, conveyor belts, hospital fabrics, etc. The ability to engineer cost-effective fabrics through one or several nonwoven production processes have allowed for rapid growth of nonwoven textiles in recent years. The technology for nonwoven production includes filament or staple fibers processed through a dry or wet-lay sheet formation step and bonded by thermal, mechanical or chemical means. Laminations of nonwovens to other nonwovens, film sheets or traditional woven or knitted textiles are often still classified as nonwovens.
One of the major nonwoven bonding methods is to treat a staple or filament fiber sheet with an emulsion polymer. When the emulsion is dried or otherwise reduced (coacervation) the polymer forms intimate bonding of the fibers. This polymer deposition modifies the strength, stiffness, environmental resistance, elongation and many other characteristics of the fiber fabric sheet. The fiber can be from a great variety of compositions, e.g. rayon, wood pulp (cellulose), cotton, nylon, polyester, glass and graphite. In the case of carded staple fiber the polymer usually contributes most of the strength and toughness character in the resulting nonwoven. In wet-laid wood pulp fiber products the polymer is able to provide the nonwoven with strength and resistance to aqueous and solvent environments which the untreated nonwoven would not have. In glass mat nonwovens traditionally bonded with a urea-formaldehyde resin, addition of emulsion polymers alters the toughness of the resulting nonwoven. Even in filament or staple fiber nonwovens which are bonded by mechanical (i.e. needle punching) or thermal (e.g. spun bonded) techniques and are formed into useful nonwoven fabrics without a chemical treatment, an additional application of an emulsion polymer can enhance or produce other valuable characteristics such as stretch resistance or non-slip character.
A great variety of emulsion polymers have been used to treat nonwovens. Traditional polymer compositions have been based on: acrylate and methacrylate ester copolymers; styrene-acrylate ester copolymers; styrene-butadiene copolymers; acrylonitrile copolymers of the above; vinylacetate polymers; vinylacetate-acrylate ester polymers; vinylacetate-ethylene copolymers; vinyl chloride polymers; vinyl chloride-ethylene copolymers and vinyl chloride-acrylate ester copolymers. All the above emulsion polymers have found use in nonwoven fabrics based primarily on the particular characteristics which the polymer can contribute to the nonwoven. Some are used because they simply contribute strength at the lowest cost level. Particular examples include (1) the use of an acrylate ester copolymer to bond polyester staple fiber for quilt stuffing and insulation; (2) the use of a vinylacetate-ethylene copolymer to give wet strength to wood pulp nonwovens used as paper towels; (3) the use of a vinylchloride based polymer to bond staple polyester fibers for flame retardant filter media; and (4) the use of a styrene-butadiene copolymer to bond high denier nylon fibers for extra tough floor polishing fabrics.
Another area in which polymer emulsions are useful is in industrial and household adhesives especially as bases for white glues, wood adhesives, packaging adhesives, film and foil adhesives and pressure sensitive and contact adhesives.
Adhesives can be prepared from a wide variety of synthetic organic polymers. Often these are blended to provide adhesive compositions displaying specific properties desired by the user. Adhesives containing vinyl acetate emulsions and copolymers thereof possess excellent adhesion to many porous and nonporous substrates such as paper, wood, metal, foil, plastic, ceramic, cloth, felt, leather, cork, glass and the like. Often such emulsions can be used with little, if any, modifications. Sometimes, however, it is necessary to alter either their physical properties and/or their application characteristics. To that end thickeners, plasticizers, tackifyers and other polymer emulsions are often added.
Wood adhesives generally contain polymer emulsions, primarily polyvinyl acetate emulsions, which have largely replaced the traditionally used wood adhesives which were based on animal glues. The polyvinyl acetate emulsions can be used as is, especially if polyvinyl alcohol is present as the emulsifier. Often additional polyvinyl alcohol is also added later to increase the tack and the heat resistance of the adhesive.
Laminating adhesives are used to produce composites of plastic films consisting of polyethylene, polypropylene, polyvinylidene chloride, polystyrene, polyester, and polyvinyl alcohol-ethylene films. The laminates are often used in foodpackaging applications. Laminating adhesives are also used to bond polyvinyl chloride films to wood to form decorative panels. Many laminting adhesives consist of plasticized homopolmer- and copolymer emulsions of vinyl acetate, such as vinyl acetate-ethylene copolymer emulsions or vinyl acetate-butyl acrylate copolymer emulsions.
Packaging adhesives are used in the production of papercartons and plastic bags for food packaging, corrugated cardboard boxes for general packaging use, blister packages and the like, because they combine excellent specific adhesion with ease of use on high speed packaging machinery. Here also homo-and copolymer emulsions of polyvinyl acetate are widely used.
Pressure sensitive and contact adhesives are used on pressure sensitive tapes, to adhere labels to metal and glass objects, such as cans and bottles, to laminate plastic surfaces to wood, and like applications. These adhesives often are based on acrylate copolymer emulsions, vinyl acetate copolymer emulsions, styrene-butadiene and chloroprene emulsions, and the like.
Although copolymerization of vinyl acetate with acrylates, ethylene, vinyl chloride and maleate- and fumarate esters of lower alcohols can provide many superior polymers when they are used as adhesive bases, there remains the problem of not being able to readily copolymerize styrene, a very economical monomer, and acrylonitrile, methyl methacrylate, or chloroprene with vinyl acetate, alone or in combination with ethylene, in order to increase the modulus at elevated temperatures of the resulting polymer. One of the disadvantages of vinyl acetate polymers in general is its lack of hardness at elevated temperatures, that is, the vinyl acetate polymers soften too readily when the temperature of use is increased even modestly, for example, to 50.degree. C. The reason for this thermoplasticity at elevated temperatures is the relative low glass transition temperature of polyvinyl acetate of about 30.degree. C. It has long been desired to raise the modulus at elevated temperatures of vinyl acetate homo-and copolymers, but no economical comonomer to accomplish the hardening of vinyl acetate polymers is commercially available. Styrene would be a very desirable comonomer, because, besides having excellent physical properties such as clarity and stiffness at elevated temperatures, it is also a very economical comonomer. It is, therefore, desirable to use as much as possible of styrene and the other low cost monomers together with vinyl acetate in adhesive formulations.
The polymer emulsion prepared by the process of this invention provides a binder compound for fiberfill which provides improved resiliency and loft recovery to the bonded fiberfill products. This polymer emulsion is useful in bonding textile fibers in a fiberfill product or in any nonwoven product or even any traditional woven or knitted textile fabric. The polymer emulsion can also be used as a binder of fabric or fibers for other nonwoven products including insulation, filters, construction fabrics, roofing materials, paper towels, carpets and other nonwoven fabrics. In addition, the polymer emulsion is useful in preparing superior and economic adhesive bases, for use in household and industrial applications.